REPRODUCTION 505 



the continuance of the race is specially provided for by setting 

 apart certain cells which shall have this one property cells 

 whose duty is to the species and not to any one representative of 

 it an essentially altruistic element in the otherwise egoistic whole. 



Sexual Reproduction. In some cases, especially among insects, 

 the specialized reproductive cells can develop, each for itself, 

 under suitable conditions, and give rise to new individuals; such 

 a mode of reproduction is called parthenogenesis: but in the major- 

 ity of cases, and always in the higher animals, this is not so; the 

 fusion of two cells, or of products of two cells, is a necessary pre- 

 liminary to development. Commonly the coalescing cells differ 

 considerably in size and form, and one takes a more direct share 

 in the developmental processes; this is the egg-cell or ovum; the 

 other is the sperm-cell or spermatozoon. The fusion of the two is 

 known as fertilization. Animals producing both ova and sperma- 

 tozoa are hermaphrodite; those bearing ova only, female; and those 

 spermatozoa only, male: hermaphroditism is not found in Verte- 

 brates, except in rare and doubtful cases of monstrosity. 



Accessory Reproductive Organs. The organ in which ova are 

 produced is known as the ovary, that forming spermatozoa, as 

 the testis or testicle; but in different groups of animals many addi- 

 tional accessory parts may be developed. Thus, in all but the 

 very lowest Mammalia, the offspring is nourished for a consid- 

 erable portion of its early life within the body of its mother, a 

 special cavity, the uterus or womb, being provided for this purpose : 

 the womb communicates with the exterior by a passage, the va- 

 gina; and two tubes, the oviducts or Fallopian tubes, convey the 

 eggs to it from the ovaries. In addition, mammary glands provide 

 milk for the nourishment of the young in the first months after 

 birth. In the male mammal we find as accessory reproductive 

 organs, vasa deferentia which convey from the testes the seminal 

 fluid containing spermatozoa; vesiculce seminales (not present in 

 all Mammalia), glands whose secretion is mixed with that of the 

 testes or is expelled after it in the sexual act; a prostate gland, 

 whose secretion is added to the semen; and an erectile organ, the 

 penis, by which the fertilizing liquid is conveyed into the vagina 

 of the female. 



The Male Reproductive Organs. The testes in man are paired 

 tubular glands, which lie in a pouch of skin called the scrotum. 



